German physicist Johann Wilhelm Hittorf studied under Julius Plücker, and became the first scientist to calculate the electric capacity of charged atoms and molecules (ions). He noted that in vacuum tubes, energy rays extend from a negative electrode, making flickers of light as the rays struck the tubes' glass walls. He improved the method of creating a vacuum inside tubes, and his experiments showed that these rays -- later termed "cathode rays" by Eugen Goldstein -- could escape if tiny cracks formed in the tube's walls, and behave differently under the effects of magnetism. When his works were translated into English, Hittorf was sometimes cited as John William Hittorf.